1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a printing machine. In particular, the present invention relates to an image forming apparatus in which an image bearing belt such as an intermediate transfer belt, and a transfer belt such as a secondary transfer belt, are driven while exhibiting contact portions being in contact with each other.
2. Description of the Related Art
In connection with an image forming apparatus employing an intermediate transfer belt bearing a toner image, there exists a configuration which adopts a transfer belt configured to bear and convey a recording material in order to achieve an improvement in terms of conveyance property for the recording material, which can be of various types (Japanese Patent Application Laid-Open No. 2007-11107).
The above-described intermediate transfer belt and transfer belt, which are endless belts, are driven while suspended between a plurality of rollers including a driving roller. Depending upon the outer diameter precision of the rollers and the alignment precision between the rollers, such belts involve an issue referred to as belt deviation, in which the belts are deviated toward one edge direction during running.
As a method of solving the issue of belt deviation, Japanese Patent Application Laid-Open No. 9-16944 discusses a deviation control method using a steering roller. According to the method, one of stretching rollers is used as a steering roller which is supported so as to be capable of freely changing axial alignment, and controlled by an actuator such as a motor. FIG. 15 is a control diagram illustrating this method, and according to a deviation E of an actual measurement value PV of the edge position obtained by an edge detection sensor from a target position SV of an endless belt edge, a control controller inputs a predetermined command value MV to a motor driver to drive the steering motor. Accordingly, the axial alignment of the steering roller is changed, resulting in a change in the edge position of the endless belt to effect a feedback control.
However, in the method discussed in Japanese Patent Application No. 9-16944, when the suspension orientation of the belt is changed, the belt conveyance direction is also changed.
FIG. 11 illustrates a generally adopted suspension layout for a belt 114. In this example, the belt is suspended on four rollers. In the drawing, a steering roller 113 is shaded; for the sake of convenience in illustration, the other rollers will be referred to as suspension rollers 111 and 112 and a driving roller 110. The belt 114 is formed of a material exhibiting high Young's modulus, and its expansion/contraction is substantially negligible. When the three rollers other than the steering roller 113 are fixed in position, the layout range for the steering roller 113 is restricted to a range satisfying the condition in which the sum total of L1 and L2 in FIG. 11 is fixed (L1+L2=constant); in other words, it is restricted to an elliptical path C whose focuses are the suspension rollers 111 and 112.
FIG. 12 illustrates the case where deviation control is effected, in this case, the steering roller 113 tries to change the axial alignment in the direction of an arrow S in the drawing by an actuator (not illustrated). FIG. 12 is a sectional view of the suspension layout, thus, specifically, the steering roller 113 tries to effect a change in suspension orientation to an inclination such that the front end thereof comes to a position 113F and that the rear end thereof comes to a position 113R. In reality, however, due to the restraint to the above-described elliptical path C, the front and rear ends of the steering rollers are respectively corrected to positions 113F′ and 113R′. The steering roller 113 also serves as a tension roller applying a desired tension to the endless belt 114 by an urging unit 120 such as a spring, and correction is effected through the expanding/contracting action of the urging unit 120. The change in axial alignment resulting from this correction is the change in the belt conveyance direction.
FIGS. 13 and 14 illustrates the suspension layout (FIG. 12) as seen from above, and illustrate a traction surface pulled by the steering roller 113. Here, the traction surface is a belt surface suspended between the steering roller and the suspension roller such that the steering roller is situated on the downstream side in the belt moving direction. The endless belt 114 is driven in the direction of an arrow V in the drawings; the solid line indicates the suspension orientation at time t, and the dashed line indicates the suspension orientation at time t+Δt. The edge position of the belt 114 is measured at two measurement points M1 and M2 in the conveyance direction (the conveyance speed is one effecting movement through a distance corresponding to the distance between the points M1 and M2 during the time Δt.) In FIG. 13, it is assumed that the steering roller 113 is inclined in the direction S (See FIG. 12), and the belt 114 is conveyed in the X-direction in a suspension orientation at an inclination angle γ. At this time, it is thought that the edge position is displaced in the Y-direction at the measurement points M1 and M2, that is, belt deviation is generated. However, tracing of a mass point Pt, which is an arbitrary point on the traction surface at the time t, shows that, at the time t+Δt, the mass point Pt is situated at the position Pt+Δt to be attained through advancing straight ahead in the X-direction, which means the mass point itself has made no displacement in the Y-direction.
In reality, however, the steering roller 113 is inclined in the direction S, and is corrected to the elliptical path, so that, as illustrated in FIG. 14, there are generated two changes: the suspension orientation of the inclination γ and the conveyance direction of the inclination β. Thus, between the time t and the time t+Δt, there are generated not only in the displacement in the Y-direction at the measurement points M1 and M2, that is, the belt deviation, but also a displacement in the Y-direction of the mass point Pt itself. As can be seen, assuming that the mass point Pt is an image (dot) formed on the traction surface, positional deviation is generated in the main scanning direction (Y-direction) in FIG. 14, and, assuming that the measurement point M1 is an image forming portion of a first color and that the measurement point M2 is an image forming portion of a second color, the difference in positional deviation appears in the form of color misregistration.
As described above, on the traction surface side of the steering roller, there exists a mass point displacement in the thrust direction accompanying steering control. Generally speaking, in the case of a two-axis suspension configuration, the concept of an elliptical path is not applicable, so that the mass point displacement does not easily lead to a problem. However, in the case of the secondary transfer belt as discussed in Japanese Patent Application Laid-Open No. 2007-11107, in which there is a need for a difference in outer diameter, a two-axis suspension configuration may involve a similar problem.
More specifically, the smaller the diameter of a separation roller 181, the more improved is the separation performance; when the difference in outer diameter between the two axes is increased as illustrated in FIG. 18A, the large diameter side roller 180 becomes equivalent to two rollers 183 and 184 close to each other as illustrated in FIG. 18B. As a result, even in the case of a two-axis suspension configuration, when a secondary transfer roller is used, mass point displacement on the traction surface due to the steering roller constitutes a problem.
In the case of a configuration in which both the transfer belt and the intermediate transfer belt receive an external force in the thrust direction, adopting a configuration in which the traction surface of one belt is allowed to come into contact with another belt results in a disturbance in which the other belt receives an external force in the thrust direction, and in which steering control of the one belt causes deviation in the other belt.